Experiments are planned to determine the neural systems and mechanisms underlying three elementary forms of behavioral plasticity: habituation (a decrease in response to repetitive stimulation); sensitization (an increase in response to a repetitive stimulus); and the "prepulse inhibition" or recovery cycle effect of the acoustic startle reaction in rats (the phenomenon in which a single stimulus alters responsiveness to subsequent stimuli for a limited period of time). Activity of single cells will be recorded from the reticular formation, frontal cortex, and hippocampus of acutely prepared as well as chronically implanted rats to determine the nature of plasticity in these structures within a single habituation training session, as well as across days (i.e., short-term vs. long-term habituation). Habituation of single neuron activity will be compared to habituation of behavioral responses in the intact behaving animal. Modulaton of reticular neuronal activity, particularly as it may be related to the modulation of the amplitude of and habituation of the acoustic startle response by frontal cortex and hippocampus, will be investigated.